1. Field
The present invention relates to a radial tire intended to be fitted to a heavy vehicle of construction plant type.
2. Description of Related Art
Although not restricted to this type of application, the invention will be more particularly described with reference to a radial tire intended to be mounted on a dumper, a vehicle for transporting materials extracted from quarries or open cast mines. The nominal diameter of the rim of such a tire, within the meaning of the European Tire and Rim Technical Organisation (ETRTO) standard, is a minimum of 25″
The following definitions apply in what follows:    “Meridian plane”: a plane containing the axis of rotation of the tire.    “Equatorial plane”: the plane passing through the middle of the tire tread surface and perpendicular to the axis of rotation of the tire.    “Radial direction”: a direction perpendicular to the axis of rotation of the tire.    “Axial direction”: a direction parallel to the axis of rotation of the tire.    “Circumferential direction”: a direction perpendicular to a meridian plane.    “Radial distance”: a distance measured perpendicular to the axis of rotation of the tire and from the axis of rotation of the tire.    “Axial distance”: a distance measured parallel to the axis of rotation of the tire and from the equatorial plane.    “Radially”: in a radial direction.    “Axially”: in an axial direction.    “Radially on the inside, or respectively radially on the outside”: which is situated at a smaller or greater radial distance, respectively.    “Axially on the inside, or respectively axially on the outside”: which is situated at a smaller or greater axial distance, respectively.
A tire comprises two beads which provide the mechanical connection between the tire and the rim on which it is mounted, the beads being joined respectively by two sidewalls to a tread intended to come into contact with the ground via a tread surface.
A radial tire more particularly comprises a reinforcement, comprising a crown reinforcement, radially on the inside of the tread, and a carcass reinforcement radially on the inside of the crown reinforcement.
The carcass reinforcement of a radial tire for a heavy vehicle of construction plant type usually comprises at least one carcass reinforcement layer made up of metallic reinforcing elements coated in a polymer coating material. The metallic reinforcing elements are substantially parallel to one another and make an angle of between 85° et 95° with the circumferential direction. The carcass reinforcement layer comprises a main part, joining the two beads together and wrapped, within each bead, around a bead wire. The bead wire comprises a circumferential reinforcing element, usually made of metal, surrounded by at least one material, which, and this list is not exhaustive, may be made of polymer or textile. The carcass reinforcement layer is wrapped around the bead wire from the inside towards the outside of the tire to form a turn-up having an end. The turn-up, within each bead, allows the carcass reinforcement layer to be anchored to the bead wire of the bead.
Each bead comprises a filling element which extends the bead wire radially outwards. The filling element is made of at least one polymer filling material. The filling element is generally made of a radial stack of at least two polymer filling materials which are in contact along a contact surface that intersects any meridian plane along a meridian line. The filling element axially separates the main part from the turn-up.
A polymer material, after curing, is mechanically characterized by tensile stress—deformation characteristics which are determined by tensile testing. This tensile testing is carried out by a person skilled in the art, on a test specimen, according to a known method, for example in accordance with international standard ISO 37, and under standard temperature (23+or −2° C.) and moisture (50+or −5% relative humidity) conditions defined by International Standard ISO 471. For a polymer compound, the elastic modulus at 10% elongation, expressed in mega pascals (MPa), is the tensile stress measured for a 10% elongation of the test specimen.
A polymer material, after curing, is also mechanically characterized by its hardness. The hardness is notably defined by the Shore A hardness determined in accordance with standard ASTM D 2240-86.
When the vehicle is being driven along, the tire, mounted on its rim, inflated and compressed under the load of the vehicle, is subjected to bending cycles, particularly in its beads and its sidewalls.
Considering that a bead behaves mechanically in bending like a beam the respectively outer and inner axes of which are the main part and the turn-up part, the turn-up, subjected to the bending cycles, undergoes compressive deformations likely to lead to its fatigue failure and therefore to a reduction in the endurance of the bead and in the life of the tire.
Document EP 2 216 189 describes a tire bead the endurance of which is improved by reducing the compressive deformations in the turn-up when the bead bends on the rim when in use. This objective is achieved by a turn-up which is such that the distance between the turn-up and the main part decreases continuously, radially towards the outside, from the bead wire, as far as a minimum distance and then increases continuously as far as a maximum distance. The turn-up extends radially on the outside of the point of the turn-up that corresponds to the maximum distance between the turn-up and the main part.
The bending cycles also lead to stresses and deformations, mainly in shear and in compression, in the polymer filling material, because of the bending of the bead on the rim flange.
In particular, at the surface of contact between two polymer filling materials, the bending cycles give rise to cracks which spread through the radially outermost polymer filling material and are likely, over time, to lead to damage to the tire that entails the tire being replaced.
According to the inventors, the cracks start as a result of the stiffness gradient between the radially innermost polymer filling material in contact with the bead wire and the polymer filling material which is radially on the outside of it and adjacent to it along the contact surface Imperfect cohesion between the two polymer filling materials, along their contact surface, is a factor initiating cracking.
The rate at which the cracks spread is dependent firstly on the amplitude and frequency of the stress deformation cycles and secondly on the respective siffnesses of the polymer filling materials. By way of example, the elastic modulus at 10% elongation of the radially innermost polymer filling material in contact with the bead wire may be equal to 3 times the elastic modulus at 10% elongation of the polymer filling material radially on the outside of and adjacent to it.
The inventors have set themselves the objective of increasing the endurance of the beads of a radial tire for a heavy vehicle of construction plant type by reducing the cracking that starts at the surface of contact between a radially innermost first polymer filling material in contact with the bead wire and a second polymer filling material radially on the outside of the first polymer filling material.